We use them indoors as our studio lights behind an umbrella or soft box. At weddings or any shoot on the run, we always have one on a Voice Activated Lightstand
Nikon's Speedlights are powerful and versatile light sources, and with a little practice, you can achieve some amazing results!
There appears to be some confusion about the basic modes of the Speedlights and when to use them. I took the following the examples to try to explain. (please note I'm using a Nikon D3 which is capable of high-speed sync outdoors).
First, the basic "TTL" mode. This mode attempts to use the flash to balance the entire scene. This is a mode best used indoors, or a location where the natural lighting is even. Here's an example of what happens with TTL mode outdoors. This shoot is taken outdoors, with the subject in full shade, (85mm, Aperture Priority, f/2.0, 1/2000, SB-900 TTL EV 0):
Yuck! This is typical in what happens in TTL mode outdoors. For quick improvement, we can switch the Speedlight to the TTL-BL mode, which stands for Balanced Fill Flash. In this mode, the flash uses the camera's exposure information to try to provide a good balance between the foreground and background exposure. Here's the same shot again with the only change the switch to TTL BL mode (85mm, Aperture Priority, f/2.0, 1/1600, SB-900 TTL-BL EV 0)
Better! Without any other manual adjustments, the camera and speedlight gave us a pretty well balance image. We can further refine the image by adjusting the camera exposure bias or the flash bias. In this example, I've put the camera into Manual mode, so I have full control over the exposure of the background. Then I dialed the compensation on the SB-900 down EV -3.0. (85mm, Aperture Priority, f/2.0, 1/2500, SB-900 TTL-BL EV -3)
In this image, you can barely tell a flash was used at all, and you also get greater saturation in the background. It just provided gentle fill flash without providing that obvious punch of on-camera flash. A little greater refinement to EV -2.7 or -2.3 might suit individual tastes.
While I'm not trying to teach a complete tutorial on Nikon Speedlights, I am trying to encourage you to grab a friend, go outside, and give it a try!